The present invention relates to a wafer inspection technique using a charged particle beam and, more particularly, to an inspection technique using a charged particle beam such as an electron beam for detecting a foreign matter or a defect on a wafer such as a semiconductor wafer having a fine circuit pattern.
As a method of evaluating a semiconductor waver having a fine circuit pattern with an electron beam, a technique of conducting a higher-precision inspection of higher throughput adapted to the larger diameter of a wafer and a finer circuit pattern is being practically used. For example, as disclosed in Japanese Patent Application Laid-Open No. H06-139985, a method of conducting an inspection for a defect by using contrast of a secondary electron beam generated due to surface potential variations is known.
In a method of detecting an electric defect from voltage contrast, an electron beam is emitted before an inspection to preliminarily positively or negatively charge the surface of a wafer, and a secondary electron image is acquired, thereby enabling voltage contrast to be increased. An example of a method of positively charging the surface of a wafer is disclosed in Japanese Patent Application Laid-Open No. 2000-208085, where a pattern such as a contact hole in which a plug is buried is inspected by irradiating the surface of a wafer with an electron beam to positively charge the surface and, after that, voltage contrast is obtained.
Although the method of conducting an inspection on a circuit in which a plug is buried is described in the above publications, a method of conducting an inspection on the bottom of a pattern having very a large step such as a hole after dry etching is not described.
On the other hand, a technique of observing the bottom of a hole pattern from an image of secondary electrons discharged when the hole pattern is irradiated with an electron beam is known. By conventional scanning electron microscopes, however, it takes time to observe an object in a limited field of view at a high scaling factor. It is therefore impossible to observe the entire surface of a wafer and detect a defect.
An example of an inspection method of negatively charging the surface of a wafer and detecting a contact hole with an open contact failure is disclosed in the above-described Japanese Patent Application Laid-open No. H06-139985 in which the surface of a wafer is negatively charged by being irradiated with an electron beam with low energy and, after that, a secondary electron image is obtained. In the method, by using the fact that when a residual film exists on the bottom of a hole, the potential of the opening is changed by the residual film, and the diameter of the hole becomes seemingly small, a hole with an open contact failure is detected.
According to the method, however, an object in a limited field of view is observed at a high scaling factor over long time, and it is impossible to observe the entire surface of a wafer to detect a defect. Since an electron beam is preliminarily emitted to negatively charge the surface of a wafer and, after that, a secondary electron image is captured and further, since irradiation electron energy used to negatively charge the surface of a wafer and that used to acquire a secondary electron image are largely different from each other, it is difficult to set an electron beam optics unit and an inspection cannot be efficiently conducted on the entire surface of a wafer. Since the wafer has to be irradiated with an electron beam twice or more, the whole surface of a wafer cannot be efficiently inspected.
A conventional inspection system using an electron beam has problems as described below.
In a conventional inspection system using an electron beam, a defect is detected from contrast obtained due to potential variations which occur on a wafer having a circuit pattern. However, it is difficult to detect the state of the bottom of a pattern having a large step, such as a contact hole with an open contact failure, with high sensitivity by detecting a secondary electron signal from the bottom portion of the pattern. Particularly, most of secondary electrons from the bottom of a hole pattern having a high aspect ratio are hindered by side walls and cannot be detected. It is therefore difficult to detect a hole pattern with an open contact failure.
By a conventional scanning electron microscope, although the shape of the bottom of a hole pattern and a foreign matter can be detected, it is difficult to detect, for example, a hole having an open contact failure. By the conventional scanning electron microscope, observation is made at high a scaling factor with high spatial resolution. Consequently, scan speed is low, the scan range is narrow, and it is impossible to scan a large area such as a wafer required for a defect detector at high speed.
In the method of negatively charging the surface of a wafer and detecting a contact hole with an open contact failure, the electron beam accelerated to low speed is emitted to negatively charge the surface of the wafer and, after that, a secondary electron image is captured. However, in the method, due to a large difference between the energy of the electron beam emitted to negatively charge the surface of the wafer and that of the electron beam emitted to obtain a secondary electron image, it is difficult to emit the electron beams by using the same electron source. Further, the electron beam is emitted once to negatively charge the surface of the wafer and, after that, the secondary electron image is observed at a high scaling factor with high spatial resolution. Consequently, the scan speed is low, the scan range is narrow, and it is impossible to scan a large area such as a wafer at high speed required for the defect detector. Thus, the entire surface of a wafer cannot be efficiently inspected.
Further, in the conventional apparatuses, the surface of a wafer is negatively charged and the opening is evaluated. Consequently, according to the kind and material of a semiconductor circuit pattern, the kind of a semiconductor device which can be evaluated is limited. There is a problem such that sensitivity of detection of a hole with a contact hole failure varies according to the kind of a circuit pattern.
It is therefore an object of the invention to provide, as a technique solving the problems, of conducting an inspection of a wafer being subjected to a semiconductor manufacturing process, a wafer inspection system and a wafer inspection process for inspecting a pattern having a large step such as a hole pattern for a defect at high speed and with high precision. Another object of the invention is to provide a technique contributing optimization of a semiconductor manufacturing process by using defect information such as an open contact failure of a hole. Further another object of the invention is to provide a technique contributing to increase reliability of a semiconductor device by detecting an abnormal process at an early stage and taking a measure in the control of a semiconductor manufacturing process.
First, the method of carrying out an inspection by positively charging the surface of a wafer will be described. The principle of the invention in which a secondary electron emitted from the bottom of a hole is detected will be described by referring to FIGS. 2A and 2B.
An image of a hole pattern is captured by a conventional inspection system using an electron beam. For example, when the irradiation energy of the electron beam is 500 eV (electron volts) and the aspect ratio is 4 or less, secondary electrons 34 exhausted from the bottom of a hole are easily detected. An open hole is observed light and a hole with an open contact failure is observed dark. However, when the aspect ratio is high, as shown in FIG. 2A, most of the secondary electrons 34 from the bottom of the hole are interrupted by side walls 35. Consequently, the open hole is also observed dark, so that a hole with an open contact failure cannot be detected.
To deal with the problem, means for preliminarily irradiating the surface of a wafer with an electron beam or the like to charge the surface 36 of the wafer to a desired voltage before capturing a secondary electron image used for inspection is provided. As shown in FIG. 2B, when the surface 36 of the wafer 36 is positively charged, a part of the secondary electrons 34 emitted from the bottom of the hole is accelerated upward and can be efficiently detected. Further, the secondary electrons 34 emitted from the bottom of the hole can be accelerated. Therefore, when the secondary electron 34 emitted from the bottom of the hole collides with the sidewall 35 of the hole, a secondary electron is further emitted from the side wall 35. A part 37 of the secondary electrons emitted from the side wall 35 is accelerated toward the opening and can be detected by a detector. As a result, the secondary electrons 35 emitted from the bottom of a hole of high aspect ratio can be pulled out and detected, and information such that the hole is open or has an open contact failure can be obtained. At this time, according to the voltage for charging the surface 36, the aspect ratio and the hole diameter by which the hole can be detected are determined. Therefore, means for controlling the surface 36 of a wafer to a desired charging voltage is provided.
The voltage of charging the surface 36 of a wafer at the time of detecting whether a hole having a high aspect ratio is open or has an open contact failure is determined by the aspect ratio, hole diameter, and kind and thickness of an insulating film around the hole. For example, to detect whether a hole having an aspect ratio of 10 formed in a silicon oxide film is open or has an open contact failure, it is necessary to charge the surface 36 of the wafer to 5 V or higher. Therefore, means for determining a charging voltage with reference to a prestored database is provided. In the invention, for a hole pattern having a step, it is desirable to set the charging voltage of a wafer in a range from 5 V to 50 V.
A method of charging the surface 36 of a wafer to a desired positive voltage will now be described. When the surface of a wafer is a silicon oxide film or a insulating film made of an organic material, as an electron beam 38 for charging, an electron beam is emitted with irradiation energy in the range from 100 eV to 1000 eV so that secondary electron discharge efficiency becomes 1 or higher. An electron beam optics unit for charging a wafer can be also used as an electron beam optics unit used to capture an inspection image. Also, means for controlling the voltage for charging the surface 36 of the wafer by applying an optimum voltage to an electrode 32 mounted on the top face of the wafer and generating an electric field on the top face of the wafer is provided.
To inspect a large area of a wafer or the like at high speed, as the electron beam 38 for charging a wafer, an electron beam having spatial resolution which is lowered as compared with that for capturing an image can be used. Further, means is provided for efficiently positively charging the surface 36 of a wafer and conducting an inspection during movement of the wafer, which is realized by, as a method of scanning the electron beam 38 for charging a wafer, dividing the wafer into a plurality of inspection areas and alternately performing charging of the wafer and capturing of a secondary electron image.
A method of inspecting a hole by using the negative charging will now be described. The principle of detecting a defect by using the negative charging will be described with reference to FIGS. 3A to 3C. When the surface of a wafer is negatively charged, as shown in FIG. 3A, open holes 40 are observed darker than an oxide film surrounding the holes. In the case of the hole with an open contact failure, an oxide film 41 residing on the bottom is negatively charged. A potential distribution in an open hole shown in FIG. 3B and that in the hole with an open contact failure shown in FIG. 3C are different from each other. In the open hole, due to a large difference between the negative charging voltage on the bottom of the hole and that on the surface 36, a secondary electron emitted from the bottom of the hole does not easily go out from the hole. On the other hand, the bottom of the hole 41 with an open contact failure is negatively charged, so that secondary electrons emitted on the bottom of a hole are detected more easily than the open hole. The closer the distance to the outer periphery of the hole is, the higher the signal intensity of a secondary electron to be detected is. As shown in FIG. 3A, the secondary electron image of the hole 41 with an open contact failure is observed in such a manner that the signal intensity of the peripheral portion of the hole 41 is higher than that of the open hole 40 and the diameter of the hole 41 is smaller than that of the open hole 40. In the case where a contact hole 42 is tapered, the diameter of the contact hole 42 is observed larger than that of the open hole 40.
At this time, by the voltage for charging the surface, the aspect ratio at which the inspection can be conducted and the thickness of a residual film on the bottom which can be detected are determined. Consequently, means for controlling the surface to a desired negative charging voltage and means for detecting the difference in dimensions of holes in a secondary electron image are provided.
In the case of charging the surface of a wafer to a desired negative voltage, when the surface of the wafer is a silicon oxide film or an insulating film made of an organic material, an electron beam having an electron beam irradiation energy of 1,000 eV or higher with which the secondary electron emission efficiency is lowered is emitted. Means for passing a heavy current sufficient to negatively charge the surface of a wafer by using an electron source for capturing an image is provided. Further, means for applying an optimum voltage to the electrode 32 mounted on the top face of a wafer to efficiently negatively charging the surface of the wafer is also provided. By the electric field generated on the top face of the wafer, the secondary electrons emitted from the surface of the wafer can be efficiently returned to the surface of the wafer. Thus, the surface of the wafer can be charged to a desired negative voltage by using the electron source for capturing an image.
As described above, by adjusting the irradiation energy of the electron beam and the voltage of the electrode 32 mounted on the top face of the wafer 36, the surface 36 of the wafer can be controlled to an arbitrary positive or negative charging voltage by the single device. As a result, irrespective of the circuit pattern and the material of a semiconductor device, various semiconductor circuits can be inspected at high speed.
A method of controlling the voltage for charging the wafer to a desired positive or negative voltage will now be described. A wafer is moved to a chip for adjusting irradiation parameters, scanned and irradiated with an electron beam for charging, and scanned and irradiated with an electron beam for capturing an image, and a secondary electron image is captured. At the time of capturing a secondary electron image, secondary electrons are detected by using an energy filter 13. As the energy filter 13, an energy filter for detecting secondary electrons equal to or higher a threshold or equal to or lower than a threshold can be used.
In the case of capturing an image by using the energy filter for detecting secondary electrons equal to or higher than a threshold, means is provided for automatically measuring the voltage for charging the surface of a wafer by repeating operations of fixing a filter voltage of the energy filter 13, capturing a secondary electron image, after that, moving the wafer to a position where precharging is performed and a secondary electron image is captured, fixing the threshold of the energy filter at the second value, and capturing a secondary electron image.
As another method of measuring the charging voltage, a secondary electron image is captured while scanning a filter voltage of the energy filter 13 and measuring the charging voltage from the captured secondary electron image. Means for measuring the charging voltage by using the methods and optimizing the energy of the electron beam for charging, the beam current, and the electrode voltage so that the surface of a wafer is charged with a desired charging voltage is provided. Further, means for optimizing the irradiation parameters of an electron beam emitted to capture a secondary electron image is provided.
After adjusting the electron beam for charging and the electron beam for capturing a secondary electron image, an inspection is actually conducted. In the case of capturing a secondary electron image at the time of an inspection, a secondary electron image can be usually captured without using the energy filter 13. However, means for capturing a secondary electron image by using the energy filter 13 in specific cases is provided. Means for optimizing the set value of the energy filter from the secondary electron image captured at the time of measuring the charging voltage is provided. By filtering the secondary electron energy and capturing an image, a defect can be detected with high sensitivity.
A mechanism for capturing a secondary electron image by using the above methods, comparing the captured secondary electron image with a secondary electron image of the same pattern captured in another area on the wafer, and thereby detecting a defect is provided. Further, a mechanism of calculating the contrast and size of a hole is provided, and a mechanism of automatically determining the kind of a defect from the contrast and size of a hole with an open contact failure is provided. Further, means for displaying a result of determination of a defect and a distribution of defects in the plane of a wafer is provided.
First, a method of detecting a defect from a secondary electron image captured by positively charging the surface of a wafer will be described. In the case where the surface of a wafer is positively charged, a secondary electron image of an open hole is observed light. In the case of a hole with an open contact failure, the oxide film residing on the bottom is charged, so that the hole is observed darker than the open hole. In the case where the contact hole is tapered, the hole is observed light and large. Means for automatically detecting a hole with an open contact failure from variations in the contrast and hole diameter and determining the kind of the defect is provided. The thicker the oxide film residing on the bottom is, the darker the hole with the open contact failure is observed. Means for calculating the thickness of the film remaining on the bottom on the basis of brightness of the hole with an open contact failure is provided.
On the other hand, a method of positively charging the surface of a wafer and detecting a short circuit of a semiconductor circuit is provided. According to the invention, the surface of a wafer is positively charged and a hole with an open contact failure can be detected. As a result, both a short circuit of a semiconductor circuit and a hole with an open contact failure can be detected by the same system.
Next, a method of detecting a failure in the case of a material of the surface of a wafer which is not easily positively charged like a polysilicon mask, or in the case where the material of the bottom of the hole does not conduct a current unlike an insulating film will be described. When the surface of such a wafer is positively charged, a secondary electron image of an open contact failure is observed dark. Since the electric field in the opening in the case of a hole with an open contact failure and that in the case of an open hole are different from each other, the hole with an open contact failure is observed darker than the open hole. Consequently, means for determining the hole with the open contact failure on the basis of the size of the hole and automatically determining the kind of the defect is provided. Means for calculating the thickness of a film residing on the bottom from the size of the hole with an open contact failure is provided.
Further, a method of determining a defect from a secondary electron image captured at the time of negatively charging the surface of a wafer will be described. In the case of negatively charging the surface of a wafer, an open hole is observed dark in a secondary electron image. In the case of a hole with an open contact failure, the oxide film residing on the bottom is charged and the electric field of the opening changes, so that the hole is observed smaller than an open hole. When a contact hole is tapered, it is observed larger than the open hole. Consequently, means for detecting a hole with an open contact failure on the basis of the size of the hole and automatically determining the kind of the defect is provided. Means for calculating the thickness of the film residing on the bottom from the size of the hole with the open contact failure is provided.
As a result, whether or not there is a defect such as a hole with an open contact failure in a hole pattern can be determined at high speed. Further, a mechanism of finely adjusting manufacturing parameters in a semiconductor device manufacturing process from the detected defect information is provided. By the mechanism, the factor of occurrence of the defect can be estimated from the kind of the defect and a wafer in-plane distribution, and the parameters of manufacturing a semiconductor device can be finely adjusted. Further, a mechanism of adding a process of reducing defects in a semiconductor device on the basis of the detected defect information to a semiconductor manufacturing process is provided. Consequently, the semiconductor manufacturing process can be optimized at an early stage from the obtained defect information such as a hole with an open contact failure. An abnormal process can be found from the kind of the wafer and the distribution of defects in the plane of the wafer at an early stage, the factor of occurrence of the defect can be estimated at an early stage, and the reliability of the semiconductor device can be increased.